These studies are directed toward three goals. The first is to develop a positive selectable marker gene, that for dihydrofolate reductase (DHFR), which may be used to facilitate DNA transfer into mammalian cells by virtue of its ability to confer resistance to methotrexate. The second is to compare the efficiency of hybrid viruses containing the DHFR gene to that of DNA precipitated by calcium phosphate technique in transforming tissue culture cells in vitro or mouse bone marrow cells in vivo, to methotrexate resistance. The third is to use viral transcription and/or replication signals to construct a DNA fragment containing coding portions of a globin gene which results in substantial globin production in bone marrow cells. To date, we have constructed three potentially functional DHFR genes by using the cloned enzyme coding sequenes derived from mRNA with SV40 and/or globin gene transcription and RNA splicing signals. One of the constructs has been put into the late region of a hybrid SV40 virus. During the late lytic infection of monkey kidney cells, two RNA species containing the DHFR enzyme coding sequences are produced and the DHFR activity on a functional assay is 6-fold higher than in control cells.